Aerospace Materials : 1 Aerospace Materials Selection of materials for a given application
Aerospace metal alloys
Manufacturing Material Selection : 2 Material Selection Operational features – principal function of the component; description of principal loads and environment
Design Criteria – most important design properties for satisfying the operational features
Manufacturing Processes – Material form and fabrication processes. The first factor to be considered in selection of a material for a given component is the application. Properties for Screening/Rating Materials : 3 Properties for Screening/Rating Materials Static strength and stiffness properties
Durability and damage tolerance properties (fracture toughness, fatigue and corrosion resistance, etc.)
Physical properties (thermal and electrical conductivity, coefficient of thermal expansion)
Producibility (cost, manufacturing considerations, etc.
Availability Metal Alloys for Aerospace Application : 4 Metal Alloys for Aerospace Application Aluminum alloys
Beryllium alloys An alloy is a mixture or solid solution of two or more metals. The atoms of one replace the atoms of the other or occupy interstitial positions between the atoms. Material Forms for Metals : 5 Material Forms for Metals Sheet and plate – a rolled, flat product
Sheet thickness less than 0.250 in.
Plate thickness 0.250 in. or greater
Extrusion – uniform cross section created by forcing metal through a series of dies
Forging – shape created by plastically deforming metal by compression, usually in closed dies. Forging creates high-strength, tough part with efficient use of material.
Casting – created by solidification of liquid material in a mold Applications for Material Forms : 6 Applications for Material Forms Sheet and plate
Sheets are used in skin of fuselage, wings, control surfaces, etc.
Plates are machined to varying thickness create optimum shapes in high-cost parts
Extrusion – used for uniform cross section parts (e.g., stiffeners on spars, ribs) where higher strength is needed
Forging – nonuniform cross section parts where high-strength is needed.
Casting – lower cost parts in noncritical areas Aluminum Alloy Characteristics : 7 Aluminum Alloy Characteristics Aluminum alloys are the most widely used materials in aircraft structures.
Al alloys are easily formed and machined.
Al alloys are relatively inexpensive.
Al alloys experience a significant reduction in strength at higher temperatures, limiting their application in supersonic aircraft. Aluminum Alloys : 8 Aluminum Alloys 1000 99% elemental Al
6000 Magnesium and Silicon
7000 Zinc Aluminum alloys are identified by a four-digit numbering system that signifies the primary alloying element. Processing used to produce specific properties (such as heat treatment) are designated by a dashed suffix following the four-digit alloy, e.g., 2024-T3. Aluminum Alloys for Airframe Structures : 9 Aluminum Alloys for Airframe Structures Group 2000: Primarily in tension applications where fatigue and damage-tolerant design are critical
Lower wing skins
Pressurized fuselage skins
Standard material has been 2024-T3
Group 7000: Compression applications or where static strength is more important than fatigue or damage tolerance
Upper wing surfaces
7075-T6, especially in military jets Titanium Alloys : 10 Titanium Alloys Titanium alloys offer, with their higher strength, offer higher structural efficiencies than Al alloys.
Ti alloys are offer selected due to high temperature endurance.
Ti alloys are significantly more expensive than Al (high material cost, more difficult to form and machine).
Galvanic corrosion resistance for fastening composite
The most common alloy is Ti-6Al-4V. Steel Alloys : 11 Steel Alloys Steel contains iron with a small percentage of carbon (0.02 to 1.7%). Other alloying elements are added to achieve specific properties such as strength, toughness, or corrosion resistance.
The mechanical properties of steels can be varied significantly by heat treating.
Some steels offer very high strength.
Steel alloys are not widely used in airframe structures except where very high strength is needed. Steel Alloys : 12 Steel Alloys 4130 Cr-Mo with 0.3% C
4340 Ni-Cr-Mo with 0.4% C Steel alloys are identified by a four-digit numbering system. Standard heat treats are identified by the ultimate tensile strength, e.g., 180 ksi. The first two digits identify the primary alloying elements, while the last two signify the carbon content. AISI 4340 is used for thicker parts than 4130 because it can be heat treated to a greater depth. Mechanical Properties : 13 Most common metallic materials used for aerospace design are
Aluminum, Steel, and Titanium. The properties for these materials
are contained in MIL-HDBK-5. Typical data includes:
σtu = Ultimate Stress σcu = Ultimate Stress
σty = Yield Stress σty = Yield Stress
E = Modulus of Elasticity Ec = Modulus of Elasticity
e = Elongation
σsu = Ultimate Stress σbru = Ultimate Stress
G = Modulus of Rigidity σbry = Yield Stress Mechanical Properties MIL-HDBK-5 Terminology : 14 Strength values are reported using symbol F. For example Fty = σty
The values reported are minimum guaranteed values based on
testing multiple specimens. The statistical confidence in the values
are given using the following bases:
A Basis: At least 99 percent of all mechanical property values are
expected to fall above the specified property values with a confidence
of 95 percent.
B Basis: At least 90 percent of all mechanical property values are
expected to fall above the specified property values with a confidence
of 95 percent.
S Basis: Minimum mechanical property values specified by various
agencies. MIL-HDBK-5 Terminology Slide 15: 15 MIL-HDBK-5 Slide 16: 16 Example: Material Selection for Minimum Weight Design A commonly used criterion in selecting materials for aerospace
structures design is minimum weight. This involves selecting the
proper combination of material and design dimensions that result
in the part with minimum weight.
Consider the three loading conditions below. It is assumed that the
only design dimension to selected is thickness t. Slide 17: 17 The expressions relating the applied external loads to the induced
stresses are: Buckling: Example: Material Selection for Minimum Weight Design
(Continued) Slide 18: 18 The weight of the member can be expressed in terms of the material
density ρ and geometric dimensions:
W = L b t ρ
Solving for t from the expressions relating loads to stress and
substituting above: (buckling) Example: Material Selection for Minimum Weight Design
(Continued) Slide 19: 19 Weight comparison of different materials may be conducted using
the expressions previously derived. Axial Load: Buckling: Bending: Example: Material Selection for Minimum Weight Design
(Continued) Slide 20: 20 1.02 Example: Material Selection for Minimum Weight Design
(Continued) Material Cost : 21 Material Cost The material cost data provided in the handout are normalized based on the cost of 2024 Al sheet, which is has been widely used in the structures of existing aircraft. Fatigue Failure : 22 Fatigue Failure Nearly every component in an aircraft structure is subjected to fluctuating loads, including the loads of pressurization, takeoff, and landing.
Discontinuities such as windows, doors, and rivets cause stress concentrations which mean these areas are of particular concern.
Because materials subject to fluctuating loads fail at stresses much lower than the stresses that cause failure under static loads, fatigue behavior must be considered in selection of materials.
Fatigue behavior is expressed as a graph of failure stress as a function of cycles to cause failure (S-N curve). Typical S-N Curves : 23 Typical S-N Curves For many steels the S-N curve levels is asymptotic to a minimum value, as shown for 4130, HT to 125 ksi. Aluminum alloys do not exhibit this asymptotic behavior, as shown for Al 2024-T4. Palmgren-Minor theory Sandwich Structure : 24 Sandwich Structure The sandwich consists of a core material, typically honeycomb, between two higher strength face sheets.
The face sheets and core may be made from an aluminum alloy or a nonmetallic composite.
The face sheets possess high in-plane strength, and the core separates them to increase bending resistance.
Design of the sandwich component must consider a variety of failure modes, including shear failure of the bond between face sheets and core. Example of Typical Transport Structure Boeing 737 horizontal stabilizer assembly : 25 Example of Typical Transport Structure Boeing 737 horizontal stabilizer assembly Typical Transport Structural ComponentBoeing 737 horizontal stabilizer rib (sandwich stiffened) : 26 Typical Transport Structural ComponentBoeing 737 horizontal stabilizer rib (sandwich stiffened) Sandwich-Stiffened Structural ComponentBoeing 767 outboard aileron : 27 Sandwich-Stiffened Structural ComponentBoeing 767 outboard aileron